Techniques for demodulating FM signals have encompassed the use of frequency modulation feedback and phase-locked loop receivers, in an effort to achieve useful operation at an input signal-to-noise ratio which is lower than that at which a standard limiter/discriminator receiver can deliver a usable output. Threshold extension applications have also included the use of direct tracking filter circuits incorporating a phase comparator to directly tune a pair of narrow pass band filters, as described in U.S. Pat. No. 2,976,408. This and other implementations are further described in an article entitled "A New Threshold Extension Demodulator for FM Multiplex" by N. C. Hekimian et al, EASCON `69 RECORD`, pages 53 - 61. Such techniques have been employed for demodulation of multi channel signals and, while they provide an improvement over a conventional linear FM discriminator, they do not afford the degree of refinement sought for SCPC (single channel per carrier) FM signals.